Over the last decade, genomic imprinting has captured the interest and imagination of many mammalian geneticists because of its clear violation of classical Mendelian assumptions concerning equality of gametes and F(1) genotypes. Although genetic studies implicate the existence of imprinting at a multiplicity of loci throughout the mouse genome, to date, only three naturally-imprinted genes have been cloned -- Igf2, Igf2r, and H19 -- and the molecular mechanism of imprinting at even these loci remains elusive. Recently, two independent examples of genetic variation that appear to cause a loss of imprinting at the paternal allele of the Tme locus have been discovered. (Although the mutationally-defined Tme locus was assumed previously to be identical to the Igf2r gene, these new results suggest that this may not be the case.) One form of variation has led to the first genetic identification of a candidate Imprintor locus (named Imp-1) that may act within the male germ line prior to fertilization, and may be at least partially responsible for the trans-acting machinery that "marks" the Tme locus. The second variant suggests that the "imprinting maintenance machinery" may be transferable between homologs in somatic cells. It is proposed to take advantage of these new systems to further investigate the genetic basis of imprinting. The rationale behind this proposal is that an understanding of these variant systems will provide a better understanding of the mechanisms responsible for normal imprinting. The five overall goals can be summarized in the form of the following questions: (1) Where does the Imp-1 locus map, and does the map position provide an entry to its function? (2) Does the Imp-1 locus have a more general role in the control of imprinting at multiple loci? This question will be approached by determining whether the "non-marking" Imp-1 allele allows paternal expression of the normally imprinted H19 locus. (3) Does the Imp-1 locus function during the haploid phase of spermatogenesis? Results will provide information on the time at which the initial imprint is marked. (4) Is the expression of developmentally abnormal phenotypes attributed previously to the "somatic transfer of imprinting machinery" eliminated in conjunction with the removal of paternal imprinting at the Tme locus? Results could validate or refute the idea of a transferable imprinting machinery. (5) What is the nature of the Imp-1 gene product? A complete understanding of the Imp-1 gene and its role in imprinting can only be attained with a cloned copy of the gene "in- hand". This will be accomplished by combining a high resolution genetic map with limited YAC library walking and candidate gene identification.